Add GQA XQA and Flash workspace recipes

[Ti-Tai Wang (titaiwangms)]

Summary

Stacked on #32446, add graph-free checked workspace recipes for the CUDA GroupQueryAttention decode backends:

• reproduce XQA semaphore and multi-block scratch sizing from plain device/shape scalars;
• retain XQA RoPE Q/K and dynamic FP32 head-sink conversion allocation parity;
• reproduce Flash regular and fast-decode LSE/split-accumulator sizing;
• preserve the GQA fast-decode distinction between KV heads used for split selection and query heads used for accumulator allocation;
• fail closed for unsupported XQA cache/storage configurations.

Flash bounded-estimation constraint

Flash workspace is not monotonic in KV length. For B=1, S_q=1, two heads, head size 64, and 108 SMs, increasing KV length from 13,824 to 13,825 changes the selected split count from 54 to 28 and reduces workspace. Future Level-1/Level-2 aggregation must compute a conservative envelope or report unavailable rather than evaluating only the maximum shape.

The split heuristic matches runtime double-threshold semantics, including a regression boundary where using 0.85f would select 17 splits instead of the runtime 20.

Scope

This PR adds concrete selected-backend recipes only. It does not add MEA/unfused recipes, complete-route composition, dynamic-bound route reachability, L1/L2 adapters, cuDNN workspace queries, runtime allocation changes, or #32071 planner integration.

Validation

• CUDA provider test target built successfully.
• 201 CUDA internal tests passed; 2 unrelated Lean Attention tests skipped.
• 44 GQA preparation/XQA/Flash tests passed, including runtime-helper parity and Flash discontinuity regressions.

Tracking: #29775
Dependency: #32446

This draft temporarily targets main because GitHub cannot use a fork-only branch as the base of an upstream PR. Its diff will reduce to this commit after #32446 merges.